This invention relates to a system for diagnosing the fuel system of an internal combustion engine.
Engine fuel control systems generally include closed loop control of the air/fuel ratio of the mixture supplied to the engine. In these systems, an oxygen sensor monitors the engine exhaust gases and provides an indication of the rich or lean status of the air/fuel mixture relative to the stoichiometric ratio. Integral and proportional correction terms are generated from the oxygen sensor output and summed to generate a closed loop correction term used in computing the fuel amount injected into the engine to maintain the stoichiometric ratio. In addition to this closed loop correction term, many systems also employ a long term calibration adjustment used in computing the fuel amount injected into the engine. This long term correction term, sometimes referred to as a block learn term, provides for an open loop trim on the fuel calculation to adjust for such things as system aging, system-to-system variations or changes in vehicle operating conditions. It is typical for a separate block learn term to be generated for each of several engine operating points each of which is defined, for example, by a specific combination of engine speed and load. The learn term for each operating point is derived from the integral term portion of the closed loop correction term and is adjusted over time so that corrections made by the integral term are minimized. In other words, the integral term correction is transferred to the learn term.